1. Field of the Invention
This invention relates to adhesive bonding of copper surfaces used in applications such as multilayer circuit boards and, more particularly, to a process and a resulting composite for enhancing the adhesion of adhesives such as epoxy glass to copper surfaces.
2. Description of the Prior Art
Epoxy adhesives are widely used to adhere copper conductors, ground planes, etc., to other structural components in applications such as circuit boards and multilayer boards. Prepreg adhesive is frequently used in such applications. Prepreg adhesive is a glass fabric which is impregnated with B-staged epoxy resin. "B-stage" is an intermediate stage in the reaction of a thermosetting resin such that, when temperature and pressure are applied, the resin softens, liquifies, gels, and then cures, completing the epoxy-resin setting reaction. At this final stage, the epoxy resin in permanently hardened.
Use of the B-stage epoxy resin and the glass fabric provides an adhesive that may be easily handled, stored and cut to the proper dimensions. However, epoxy glass prepreg materials provide only marginal adhesion to copper surface areas. This marginal adhesion is attributed primarily to chemical reactions which occur at the copper-epoxy interface. First, it should be noted that epoxy glass prepreg materials are adhered to copper components using lamination techniques which involve pressure and heat and, frequently, heat cycling. Unfortunately, if such an epoxy adhesive is heated while in contact with copper, a dehydrogenation reaction takes place which reduces the polar nature of the groups on the epoxy molecule which provide adhesion. In addition, the hydrogen released by the reaction reacts with and tends to destroy any copper oxide by reducing the oxide to copper and water. This also is detrimental to the adhesion between the copper and the epoxy adhesive because copper oxide films are porous and the porosity enhances mechanical bonding between the copper and the epoxy glass adhesive. See "Preventing Delamination of Circuit Boards and Flexible Cicuits," Insulation/Circuits, pg. 1973, pg, 34-35, by Schuessler.
Numerous copper surface treatments have been attempted to enhance the copper-epoxy glass prepreg bonds, with limited success. The treatments include priming with epoxy resin, sputtering aluminum, black oxiding, and mechanical cross-hatching. As may be appreciated, it is highly desirable to have a means of enhancing the bond between copper surfaces and epoxy glass prepreg materials.